Auto merge of #19882 - aneeshusa:move-global-env-vars-to-servo, r=jdm

Move global CI environment variables to servo

Migrate RUST_BACKTRACE out of saltfs.
SCCACHE_IDLE_TIMEOUT has already been migrated.

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- [x] `./mach test-tidy` does not report any errors
- [x] These changes help with servo/saltfs#770 (github issue number if applicable).

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etc/ci/buildbot_steps.yml

@@ -1,4 +1,5 @@
 env:
+  RUST_BACKTRACE: '1'
   RUSTFLAGS: -Dwarnings
   CARGO_INCREMENTAL: "0"
   SCCACHE_IDLE_TIMEOUT: "600"